Which, as David Montgomery explains, has been a threat to civilizations for as long as there's been history to explain what happens when civilizations mistreat their soil. When dirt isn't being exhausted of the nutrients necessary to produce crops, it's being eroded faster than nature can replace it.

Writes Montgomery, "Civilization's survival depends on treating soil as an investment, as a valuable inheritance rather than a commodity -- as something other than dirt."

That sentence comes from Montgomery's recently published "Dirt: The Erosion of Civilizations," a book well worth your time since it touches on a few dozen contemporary issues including the farm bill, agricultural subsidies, ag trade, economic development for emerging economies, food-supply adequacy and -- as we shall see -- the increasing expectation that our farms will not only feed us but produce the raw materials for fuels that will power our vehicles.

Montgomery is a professor of earth and space sciences at the University of Washington; his previous book, "King of Fish: The Thousand-Year Run of Salmon," was a recommendation in one of this column's summer-business-reading lists a few years back.

The premise of the book is simple enough: "Many factors may contribute to ending a civilization, but an adequate supply of fertile soil is necessary to sustain one. Using up the soil and moving on to new land will not be a viable option for future generations....

"The only ways around the boom-and-bust cycle that has characterized agricultural societies are to continuously reduce the amount of land needed to support a person, or limit population and structure agriculture so as to maintain a balance between soil production and erosion. This presents several near-term alternatives: we can fight over farmland as the human population keeps growing and soil fertility declines, maintain blind faith in our ability to keep increasing crop yields, or find a balance between soil production and erosion."

But this is not one of those "there's no hope, we're all doomed" stories. Techniques to keep the dirt productive and in place have been known for centuries -- crop rotation, reinvigorating the soil with organic material including manure, providing ground cover to reduce wind and water erosion, low- or no-till planting techniques -- and research has demonstrated they can be adapted to modern, large-scale farming.

"If you look at the last 20 or 30 years, there's been a net positive trend in soil erosion," Montgomery says.

Evidence to support that comes from John Reganold, soil science professor at Washington State University. Eastern Washington's Palouse is one of the world's great wheat-growing regions; because of its hilly terrain and the silt-loam texture of the soil, it's also one of the nation's more erosion-prone areas.

In recent years farmers have shifted to techniques designed to minimize soil disturbance and took advantage of programs that offer incentives for conservation-banking erosion-prone lands. The result has been a reduction of the region's erosion rate to 8 to 9 tons per acre -- not good, Reganold says, certainly not to replacement rates, but better than it was (while these amounts sound like freight-train volumes of soil, Reganold says a ton per acre is about a sheet of paper in depth).

Making that kind of progress is an even bigger deal than it looks because farmers, in shifting to low- or no-tillage or organic farming, face major changes in operating philosophy, not to mention practical issues such as labor and capital requirements and methods of weed control.

But just as a few gullywasher storms can erase soil from a field in volumes measured not by sheets of paper but in reams, so, too, might the alternative-energy binge stall or even reverse the progress made on erosion control and soil preservation.

The demand for crops, particularly corn, to feed ethanol and other alternative-fuel refineries is pushing up commodity prices. In the rush to take advantage of those prices, farmers may be tempted to put any available acreage into production -- including marginal, erosion-prone lands that might otherwise be set aside in a conservation program. To reverse the progress made to date, Montgomery says, "would be troubling."

Trying to maintain some balance and caution in a (pardon the expression) land-rush environment is difficult, especially when dealing with an item to which little value or attention is normally given. "The cheapest input to agricultural systems, soil will always be discounted -- until it is too late," Montgomery writes.

But both those qualities will be necessary to ensure that American farmland is able to produce, in sufficient quantities and for the long haul, both food and fuel. Treat it like dirt, and pretty soon it won't be able to deliver either.